Here’s a look at the most important moments in Eric Holder’s tenure

Attorney General Eric Holder is resigning from his post as the longest-serving member of President Obama's Cabinet. From the "Fast and Furious" scandal, to collecting reporters' phone records, to the Defense of Marriage Act, here's a look at Holder's comments on some of the biggest controversies during his nearly six-year tenure at the Department of Justice. (Jackie Kucinich, Julie Percha/The Washington Post)

Attorney General Eric H. Holder Jr. is resigning after six years in office, President Obama is expected to announce Thursday. As the nation's chief law enforcement officer, Holder has been responsible for policing everyone from banks to drug cartels. His long tenure as the first African American attorney general has been marked by several successes as well as intense criticism from his opponents. Here is a look back at the decisions he made.

Race

Holder announced this month that the Justice Department would investigate the use of force by police in Ferguson, Mo., after the shooting death of an unarmed young black man there this summer at the hands an officer led to days of protests and occasional violence.

Tensions in the St. Louis suburb seemed to ease when Holder traveled to Ferguson, where he met with young people and community leaders. At other times, however, Holder's forthright comments about his own experiences as a black man have incited controversy, such as when he called the United States "a nation of cowards" on racial issues during the first months of the administration.

Holder's Justice Department has brought aggressive legal action against states that enacted voter-identification laws, arguing that they are merely a disguise for efforts to disenfranchise black voters. Many of Holder's critics have objected strongly to these suits. "I think it's just a misallocation of resources and enforcement priorities," said Ilya Shapiro of the Cato Institute.

The attorney general has faced unrelenting criticism for failing to bring criminal charges against any Wall Street executives in the wake of the economic meltdown. Even after the Justice Department secured multibillion-dollar settlements with JPMorgan Chase, Citigroup and Bank of America for their role in selling faulty mortgage securities, lawmakers and financial reform advocates have questioned why individuals behind such schemes have never been charged.

In defense of his agency, Holder has stressed the difficulty of bringing criminal charges against top-level executives who are rarely involved in the day-to-day operations of their firms. Prosecutors, he has said, need evidence of culpability, the kind of proof that often comes from cooperating witness or whistleblowers. Just last week, Holder called for Congress to increase the whistleblower award as an incentive for Wall Street executives to come forward with information.

Critics have accused Holder of handling Wall Street with kid gloves, a critique he fueled a year ago by telling lawmakers that some financial firms had become so large that it was difficult to prosecute them because of the potential impact on the economy. The remark became a flash point for Holder’s detractors on both sides of the ideological spectrum. Since then, Holder has tempered his argument by insisting that prosecutors need to work with regulators to hold institutions accountable without ruining their entire business.

Despite the continued cries for executives to face jail time, consumer groups have praised Holder’s Justice Department for carving out billions of dollars in relief for struggling homeowners and neighborhoods ravaged by the financial crisis in every major bank settlement. Yet many on Wall Street wondered why consumers were due anything when it was investors who were saddled with losses from the troubled mortgage securities at the heart of the deals. It is unclear whether the mortgage reductions and community stabilization funding that banks like JPMorgan agreed to had much impact, since an audit of the bank’s efforts is still forthcoming.

Drugs

U.S. Border Patrol Agent Brian Terry was shot and killed in 2010 with a weapon linked to Operation Fast and Furious, an initiative by the Justice Department to allow suspected smugglers to carry firearms into Mexico so that they could be traced to drug cartels. Holder has sparred repeatedly with Republicans in the House who have sought more information about the effort, and the courts are still considering the issue.

In general, Holder’s tenure as attorney general was marked by a change in tone on federal drug policy that saw agencies shifting from an enforcement-centered “war on drugs” approach to one with more focus on prevention and treatment.

Perhaps more significantly, Holder’s Justice Department made a conscious decision to take a hands-off approach with states that liberalized their marijuana laws, notably Washington and Colorado. It also took steps toward softening restrictions that would prevent banks from dealing with businesses involved in the sale of marijuana.

And in a recent interview with Katie Couric of Yahoo News, Holder said he thought it was time for the federal government to review the classification of marijuana as a Schedule 1 drug, and to consider decriminalizing marijuana use at the federal level.

In addition, the attorney general sought to limit prison sentences for nonviolent drug offenders. He instructed federal prosecutors not to charge them with crimes that carried a mandatory minimum sentence last year. "Too many Americans go to too many prisons for far too long, and for no truly good law enforcement reason," he said. With Holder's support, the national sentencing guidelines on sentencing drug users were revised this year to shorten prison terms, allowing around 50,000 prisoners to petition a judge to review their sentences.

Holder announced just this week that the federal prison population had declined slightly for the first time in decades, although the total number of inmates including those in state prisons and local jails increased.

Spying on Americans

Holder defended his department's decision to seize call records for about 20 phones at the Associated Press last year. Investigators were trying to identify the source for an AP report on May 7, 2012, on a terrorist plot in Yemen.

Holder said the AP's journalism had put Americans at risk, but the decision was sharply criticized as an infringement on the reporters' constitutional freedoms and one that would make it more difficult for reporters to inform the public about national security using information obtained from government sources without official approval. The department also monitored James Rosen, a reporter at Fox News, suggesting that he was a criminal co-conspirator.

Under Holder, the department also declined to press charges against the CIA after Sen. Dianne Feinstein (D-Calif.) accused the agency of spying illegally on her staff and holding up the release of a classified Senate report on torture.

Immigration

Holder oversaw the federal lawsuit against Arizona that culminated in a Supreme Court decision in 2012 striking down of most the state's law on immigrants -- although the court upheld the provision requiring police officers to ask anyone who looked like they might be in the country illegally for documentation. Holder also brought cases against Alabama, Utah and South Carolina, states that had passed similar laws.

The cases were the first brought by the Justice Department against states over immigration, said Marielena Hincapié, director of the National Immigration Law Center, helping to establish immigration as an important area of civil rights.

"We really saw an attorney general and a department of justice that was willing to lead on these issues and to take risky moves," Hincapié said.

Yet she added that Holder (or his successor) still has some important unfinished business with regard to the country's immigration courts, which are overseen by the Justice Department and are overwhelmed with cases.